Flame ionization detector



Dec. 8, 19 70 I D, JENTZSCH ET AL 3,545,936

Y FLAME IONIZATION DETECTOR Filed Spt. 11 1967 I s Sheets-Sheet 1 VOL T #55 SOURCE TO HM souRCE OF HYDROGEN G55, CHAR/ER &$HN

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INVENTORS. 2461716 Jenizs'ch BY Helmui Kruger Dec. 8, 1970 D. JENTZSCH ET AL 3,545,936

' FLAME IONIZATION DETECTOR Filed Sept. 11, 1967 3 Sheets-Sheet :s

(ml/min) (ml/mm) United States Patent Int. Cl. G 1n 31/12 US. Cl. 23-254 7 Claims ABSTRACT OF THE DISCLOSURE A flame ionization detector includes an air guide member and a burner nozzle which are maintained at a same electrical potential and which are supported on, and insulated from a detector support means. The air guide member defines an internal frusto-conically shaped passageway. An orifice of the burner nozzle is positioned in the passageway in a manner for causing air to flow toward and converge near the orifice. Means are provided for conveying a combustion and a carrier gas to the burner nozzle and for introducing air for combustion between the air guide member and burner nozzle. Withthis arrangement, the dependence of the detector sensitivity upon flow rates is advantageously reduced.

The present invention relates to detectors for use with analytical instruments. The invention relates more particularly to an improved form of flame ionization detector.

It has been found that the physical size of a flame ionization detector can be reduced while simultaneously providing a desired detector linearity and sensitivity by maintaining a cup-shaped air guide member and a burner nozzle at a same electrical potential, and by extending the nozzle through a passageway in a lower portion of the air guide membenA detector of this type is disclosed and claimed in copending US. patent application, Serial No. 556,299, filed June 9, 1966 now US. Pat. 3,455,647, which is assigned to the assignee of the present invention.

In one form of this detector arrangement, the passage flares out in opposite directions from a relatively narrow constrictive area. The burner nozzle extends into the passage from below and an orifice of the nozzle is positioned beyond this narrow constriction in a manner for providing that the flame itself burns upwardly of the lower portion of the air guide member. The air guide member is then effective to concentrate an air current on the flame and thereby reduces the consumption of air and provides improved combustion. The cup-shaped configuration of the air guide member shields the flame from the remaining volume of the detector chamber and it is found that detector sensitivity increases while interfering influences are reduced. However, in order to compensate for a reduction in the range of linearity accompanying a reduction in the detector dimensions, both the air guide member and the burner nozzle are maintained at the same electrical potential.

Although a detector of this type has many attending advantages, it has been demonstrated that in this as well as other forms of flame ionization detectors, the detector sensitivity is dependent to a large extent upon the flow rates of combustion gas, air, and carrier gas.

Accordingly, it is an object of the present invention to provide an improved form of flame ionization detector.

Another object of the invention is to provide a detector adapted for maintaining a relatively constant detector sensitivity within a range of flow rates of these gases.

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A further object of the invention is to provide a flame ionization detector of relatively small dimensions having desired sensitivity and linearity and which exhibits sensitivity relatively independent of flow rates over a range of flow rates.

In accordance with a feature of the present invention, a flame ionization detector includes a burner nozzle cham ber defined by an arrangement including an electrically conductive base support member, a generally tubularshaped electrically conductive side wall member, an electrical insulating means supporting said side wall member on, and, insulating said side-wall member from said support member, and an air guide member. The air guide member includes an internal passageway having a convergent segment and terminates in an outlet aperture. A burner nozzle is supported on and electrically insulated from the base member within the chamber in a manner for providing that an orifice of the nozzle is positioned in the passageway of the air guide member. The nozzle comprises a first electrode of the detector and a second electrode thereof is spaced without the burner nozzle chamber relative to the aperture. Means are provided for establishing a low impedance conductive coupling between the air guide and nozzle members for maintaining these members at a same potential.

.In a particular arrangement in accordance with the present invention, the restriction is formed as a frustoconically shaped passageway. The burner nozzle orifice is positioned within the frusto-conically shaped segment. An extension of the linear range of measurement can be obtained when the burner nozzle also has a frusto-conical shape and forms a relatively narrow annular gap with the guide body in the area of the nozzle orifice. With an arrangement including a frusto-conical burner nozzle, there has been obtained a linear indication for currents ;12.5 10-' amperes. In contrast, a calibration characteristic for a known detector shows a substantial deviation at about half of this value. In another particular arrangement of the invention the conical angle of the burner nozzle is less than the enclosed conical angle of the portion of the air guide member.

It can be demonstrated that in a detector arrangement constructed in accordance with the features of this invention, a substantial independence of the hydrogen and carrier gas flow rates can be obtained over a relatively large range. Furthermore, the maximum sensitivity is obtained with a relatively small amount of air. It is believed that the improvement attained by the present invention can be explained by the fact that in a known flame ionization detector, the flame burns in an open space, that is, at a distance from the walls and there occurs a high turbulence at the flame feather. The detector signal is then dependent on the gas flow rates. Since the arrangement in accordance with the present invention provides an opti mum signal while requiring relatively less air, it is believed that with the arrangement according to the present invention, the turbulence is substantially reduced.

These and other objects and features of the invention will become apparent with reference to the following specifications and drawings wherein:

FIG. 1 is a diagram in sectional form illustrating a flame ionization detector constructed in accordance with features of the present invention;

FIG. 2 is a diagram in sectional form of a portion of another flame ionization detector presented for comparison with the detector of FIG. 1;

FIG. 3 is a diagram of the characteristics of the detector of FIG. 1 illustrating the relationship between the detector output signal amplitude plotted versus combustion gas flow rate for different carrier gas flow rates;

FIG. 4 is a diagram of the characteristics of the detector of FIG. 2 illustrating the relative dependence of the detector output signal amplitude on combustion gas flow rate for different carrier gas flow rates;

FIG. is a diagram of the characteristics of the detector of FIG. 1 illustrating the relationship between the detector output signal amplitude and the air supply flow rate; and

FIG. 6 is a diagram of the characteristics of the detector of FIG. 2 illustrating the dependence of the output signal amplitude on the air flow rates.

Referring now to FIG. 1, a metallic burner nozzle 10 including an orifice 11 is shown to have a frustro-conical shape. The burner nozzle 10 is connected with a threaded connecting piece 14 via an insulating member 12, which is formed of a material such as refractory material exhibiting a relatively high electrical resistance at the high operating temperatures encountered. The connecting piece 14 is screwed into a bore in an enlargement of a combustion gas and carrier gas channel 16 located in a metallic detector support member 18. The channel 16 is supplied by a source represented as 17 with hydrogen, as a combustion gas, and a carrier gas which conveys a sample from the outlet of a chromatographic separating column. This gas mixture passes through a central bore of the connecting piece 14 and of the insulating member 12 and is burned at the nozzle 10. The air required for combustion is supplied from a source 19 via a line 20 which terminates in a channel 22 in the detector base. The channel 22 terminates adjacent the connecting piece 14 on the top of the detector base.

A hollow-cylindrical metal body 24 forming a side wall of the detector is supported on and spaced from the detector base 18 by a ring of insulating material 26. The body 24 and ring 26 are secured to the detector base by means of screws 28 which extend for a distance through bores of the body 24. The heads of the screws are supported by rings of insulating material 32. A bi-metallic spring 34 is arranged in a circumferential groove of the body 24 in a circular configuration and includes leg segments extending inwardly for engaging both sides of the nozzle 10. The nozzle 10 and the body 24 are thus conductively coupled and are maintained at a same electric potential. This potential is applied to the body 24 via a terminal 35 and a body 36 referred to hereinafter. An arrangement of this general type for providing the conductive coupling is disclosed in greater detail and claimed in the referred-to copending patent application.

An air guide member 36 having an internal passage- !way including a frusto-conically shaped segment and a constrictive throat segment 38 terminating in an outlet aperture 39 is threaded on an outer surface thereof and is screwed into the hollow-cylindrical body 24. The burner nozzle 10 extends into the frusto-conically shaped segment 40 in a manner for providing that an orifice 11 thereof is positioned within the convergent segment 40. The nozzle is therefore positioned in a burner chamber which is defined by the members 18, 26, 24, and 36. The cone angle of the nozzle 10, i.e., that cone formed by the outer surface of the nozzle 10, is smaller than the enclosed angle of the segment 40. Therefore, between the air guiding body 36 and the conical nozzle 10, an air supply channel of constantly reducing cross sectional area, in the form of a cone-shaped shell, is formed. This channel includes a narrowest portion in the area of the flame feather. Adjustment of the channel is effected by altering the position of the threaded air guide member. The air guide member is then locked in the adjusted position by a screw 41.

A second conductive hollow-cylindrical member 42 is mounted on the member 24. This member 42 is closed at the top thereof by a coverplate 44. The members 42 and 44 form with the air guide member 36 a second chamber for the detector. A flame at the burner nozzle 10 burns substantially internally of the restrictor 38. An electrode 46 is positioned above the flame and burner nozzle 10. This electrode is mounted by means of an insulating member 48 and extends out of the hollowcylindrical body 42. The body is thus electrically insulated from the surrounding detector member. In order to maintain the insulating member 48 relatively cool, and the corresponding electrical resistance at a relatively high value, the insulating member is mounted in a tubular projection 50. An electrical potential is applied to the electrode 46 from a source 51 via an impedance 53. A flame ignition device, designated by the reference number 52 is also provided.

FIG. 2 illustrates a detector arrangement wherein an air guide member 54 is formed with a restrictive internal passageway 56 flaring out from a narrow throat portion both upwardly and downwardly in a frusto-conically shaped arrangement. In this detector, which is presented in order to demonstrate the comparative operating characteristics of the detector of FIG. 1, the burner nozzle 10 extends through the throat so that the flame burns freely in space upwardly of the throat portion.

The operating characteristics of the detector of FIG. 2 are illustrated in FIG. 4. Output signal amplitude is plotted versus a hydrogen combustion gas flow rate to the burner nozzle with nitrogen carrier gas flow rate as a parameter. It is seen that the output signal amplitude for a predetermined carrier gas stream exhibits a relatively large change with a change in the hydrogen stream flow rate. It can also be seen that a relatively large change in the signal amplitude occurs when the carrier gas stream N is varied.

In comparison, FIG. 3 illustrates the corresponding characteristics obtained with the detector of the present invention. In a range of hydrogen combustion gas flow rates between 35 and ml./min. and of nitrogen carrier gas flow rated between N =10 ml./min. and N =35 ml./min., the signal is substantially independent of these flow rates. In addition the signal variations are relatively small outside of this range.

FIGS. 5 and 6, are plots of the characteristics of the output signal amplitude versus the air supply for the detectors of FIG. 1 and FIG. 2, respectively. It is seen that with increasing air supply the signal increases and tends toward a saturation value. However, this saturation value with the arrangement of the invention according to FIG. 1 is attained for a smaller air flow than with the detector arrangement of FIG. 2. Therefore, in obtaining an optimum signal, smaller amounts of air can be operated with the detector of FIG. 1.

A flame ionization detector has also been operated wherein the burner nozzle is cylindrical, rather than conical as in FIGS. 1 and 2. In this test arrangement the restrictive air guide passage was formed by a disk with an aperture which, similar to 56 in FIG. 2, flares out in funnel-shaped manner upwardly and downwardly from a throat portion thereof. The orifice of the burner nozzle, in accordance with the invention, was positioned beneath this restrictor so that the flame itself burned substantially internally of this restrictor. With this arrangement a relatively constant output signal amplitude was obtained for variable carrier gas and hydrogen streams. Above a flow rate of about 200 ml./min. air current, the signal was substantially independent of the air supply. Thus, very favorable conditions were obtained with respect to the independence of the signal of the gas streams. Above this, the range of linearity was more strongly limited than with the arrangement according to FIG. 1.

From these results, it is concluded that the most favorable range of linearity is influenced by the shape of the burner nozzle, while the arrangement of the nozzle with respect to the restrictive passage, in accordance with the invention, is determinative of the independence of the signal from the gas flow rates.

Thus, an improved detector arrangement has been described which provides relatively good sensitivity and linearity while simultaneously demonstrating a substantial independence from combustion gas, carrier gas, and air flow rates over a relatively large range.

While we have illustrated and described a particular embodiment of our invention, it will be understood that various modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

We claim:

1. A flame ionization detector comprising:

a metallic support member;

a metallic side body member;

a body of electrical insulating material for supporting said side body on, and, insulating said side body member from said support member;

an air guide member supported by said side body member, said air guide member including an internal convergent passageway having an outlet aperture;

said metallic support member, said side body member,

said air guide member and said insulating body relatively positioned for forming a burner nozzle chamher;

a burner nozzle having an outlet orifice;

electrical insulating means supporting said burner nozzle in said chamber and positioning said orifice in said passageway;

means providing an electrical conductive connection between said burner nozzle and said air guide member;

means forming with said air guide member a second chamber for the detector; and,

an electrode positioned in said second chamber relative to said aperture.

2. The detector arrangement of claim 1 wherein said passageway includes a frusto-conically shaped segment and said orifice is positioned within the frusto-conically shaped segment.

3. The apparatus of claim 2 wherein said burner nozzle is frusto-conically shaped.

4. The apparatus of claim 2 including means for applying an electrical potential between said electrode and said burner nozzle.

5. The apparatus of claim 2 including means for varying the relative position of said air guide member with respect to said burner nozzle.

6. The apparatus of claim 5 wherein said means for varying the relative position of said air guide member and burner nozzle comprises a first threaded surface formed on a surface of said side body member and a second threaded surface formed on a surface of said air guide and engaging said first threaded surface.

7. A flame ionization detector comprising:

a metallic support member;

a side body member having a cylindrically shaped bore;

a hollow annular body of electrical insulating material supporting, and, insulating said side body member from said support member;

a cylindrically-shaped air guide member supported by said side body member and including an internal passageway having a frusto-conically shaped segment and an outlet aperture;

said metallic support member, side body member, air guide member and said insulating body relatively positioned for forming a burner nozzle chamber and for providing electrical connection between said air guide and side body members;

a frusto-conically shaped burner nozzle having an outlet orifice;

electrical insulating means supporting said burner nozzle in said chamber in a manner for positioning said orifice in said frusto-conically shaped passageway segment;

means providing an electrical conductive connection between said burner nozzle and said side body member;

means forming with said air guide member a second chamber for the detector;

an electrode positioned in said second chamber opposite said outlet aperture; and,

means for providing an electrical potential between said burner nozzle and said electrode.

References Cited UNITED STATES PATENTS 3,086,848 4/1963 Reinecke 23--254E 3,330,960 7/1967 Rich 23-254EX 3,372,000 3/ 1968 Gallaway et al. 23254E OTHER REFERENCES Ongkiehong, L, Gas Chromatography 1960, Proceedings of the Third Symposium, Edinburgh, June 8-10, 1960, edited by R. P. W. Scott; London, Butterworths 1960; pp. 7, 8.

MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner US. Cl. X.R. 23-232 

